Extractive distillation



Feb. 16, 1954 w. E. CATTERALL EXTRACTIVE DISTILLATION Filed NOV. 4, 1948 ,.MMBOP fllom E'. Calieralz Uglvgrzbor b5 Clbbor'rzeg ?atented Feb. 16, 1954 UNITED `SiTA'l'liS PATENT 4'ITEF'IC151 William 'CatteralhElizabetm .,N. J., .assigner -to Standard Oil Development llompan'y, -a .corporatiuno'f Delaware .Application-November 4, 19.48. .Serial.:No. V58,310

2 Claims. ..1 y This invention .relates lto almethod oi separating acetone from close boiling impurities difficult to separate by ordinary `:fractional .distillation such as aldehyde, ether, tester, .and acetal .im--

Vpurities, by means vof extractivedistillation A.el

,ploying water in controlled concentrations as .the extractive distillation solvent.

The separation 'of .pure Yacetone from .a mii:- ture of oxygenated compounds such as obtained fromhydrocarbon synthesis, hydrocarbon .oxidation or olefin hydratiomis verfydii''cultotving to the closeness Ain boilingpoint .of the oxygenated compounds to acetone. For example, an acetone cut may contain among vother components the .following .materials `Which would `be `considered tion, the crude acetone 4mixture is distilled in a Ifractional distillation Azone in which an internal liquid reflux medium having a high water con centration, for example, above -^60 =mol percent water, preferably above 9i) Ymol per cent, flows countercurrently to yvapors distilling Vfrom the mixture. The close-boi1ingneutral organi-c compounds `except valcohols are distilled and recovered overhead while the acetone and alcohols Vremain inthe aqueous reiiu-x solution removedas bottoms from the fractional distillation `zone.

To separate methanol, if present, ythe Adilute aqueous acetone bottoms that are 'freed -o'f fthe close-boiling neutral oxygenated'compound contaminants and hydrocarbons are ted into a second fractional #distillation zone wherein the organicvapors being distilled'againpass `countercurrent vto a liquid reflux rmeiiium o'f high Water concentration, 'for example, 'above 50 'moliper rcent water,.preferably.80 Lto '90 ml-per cent.

The second fractional distillation zone .contains a rectifying section in `whichthe vapors .becomericher in acetoney relative tomethanolwhile .Water .isied .to this section .to .ormin .the 'liquid 4reflux -'through Amost of this .section a `.solution .having @the .required Lwater concentration above .about .5.0 mol fpericent, preferably l80 :to f90 .mol per cent. The acetone `|vapor .is taken overhead N,while `the reflux .which is :a diluteaqueoussolu- A.tionmf methanol .is removed as bottoms. .5I/he acetone r.can be :taken :overhead sin practically anhydrous condition ,by .using -a sufficient numof plates between `.the :top ofthe fractionating towereand the Wateixinjectionipoint. .Theuseioi -water as an extractiva distillation; solvent ein the second tower is .not necessary .to lseparate any ethanol .from .the acetone, .but :it does increase the .relative ,volatility -for l.this separation. .No advantage `.by adding .Water .in the separation lof `isopropanol or .higher lalcohol .from acetone .is indicated.

The inventionmay be summarized ...briefly .in that the crude acetone, which may ormay notbe ,an .agueous solution, isisubiected .to .a distillation operation .in which .the .crude acetone solution .is .fed .to .a distillation tower atia ,point y.below .the top, .preferably at .about the .midpoint -of the .tower.andvin .which .Water is fedtothe .top ofthe ,tower l or :atxa point .near the top thereof, but in Iall cases .above the .acetone .feed plate, .in .suf-

V.cient quantity to .maintain a composition above 6,0 Emol per ,cent .water 4in the .liquidphasedn the .distillation column. 'The .operable Water concentration will vary .with .the particular .irnpurities present .in .the .acetone Yfeed; lhowever,

:the water concentration .will be .above T65) mol percent water,*pieerablyabovee .mol .per cent.

.The .Water added v.during 'the'.distillation is sufficient topermit taking overhead .all or substantially .all of the -.neutral .oxygenated compound impurities .plus (hydrocarbons present in the crude acetone feed except that any alcohols present vwill remain with the .acetone lto be sepa .rated v.as previously related.

The method ofthe invention will be explained with .reference to the .now plan diagram illusqtratediin 'the drawing'in which numeral AI representsjthecrudeacetonefeed line .to a first frac- ,.tionationtowerZ, which may be of conventional tower design. "The .required ihigh vWater concentration 'is eiiected by introductionof water into .the top l.of the .tower viajline 3. The water constitutes .an .internal .liquid .reflux descending countercurrent through .the vapors undergoing reeticationatan .upperpart .of the tower. The .vapors 4,passing .overhead .via ,line E `from ktov/.er 2 include vthe .separated impurities comprising .aldehyde, ether, ,es ter, .aceta'l and hydrocarbon impurities. .The .agueousreux reaches thebot- 5. Part of the bottoms is subjected to reboiling by indirect heat exchange with a heating medium in reboiler 6. A portion of the vapors passing overhead may be condensed in condenser` IB and refluxed to the tower via line I'I if desired, or the equivalent can be accomplished by injecting water through line 3 at a temperature below the boiling point of the mixture on the water injection plate to accomplish condensation of vapors on this plate. If the condensate forms two layers the water layer may be refluxed selectively.

The bottoms withdrawn from tower 2 via line 5 is passed onto a feed plate in a second fractionation tower I in which the vapors againpass through a rectification section above the feed plate. These vapors contact descending aqueous reiiux provided by injectionof Water into the top of tower 'I via line 8. The water is introduced in suiiicient quantity to maintain a water concentration between 50 to 99 mol per cent, prefer- 'ably 80 to 90 mol per cent, at the upper part of the rectiflying section of the'tower. Acetonerich vapors substantially free of any alcohol contaminants are withdrawn overhead from tower 'I via line 9, condensed and cooled in cooler I0 and passed to storage vessel I3 via line I2. A

portion of the acetone condensate is returned 'has external reiiux to tower 'I via line II. The

aqueous reflux which reaches the bottom of tower l is a dilute aqueous alcohol solution which is withdrawn via line I4. This bottoms liquid is subjected to reboiling by heat exchange with a ,heating medium in reboiler I5, or by the injection of `live steam. The aqueous alcohol is withdrawn via line I4 and may be subjected to furthei" separation treatment to recover the alcohol components thereof and to permit recycling the water if desired.

In the attached table of experimental relative volatility data of acetone to a number of other organic compounds boiling in the same range,

the effect of water on the various compounds can be observed. Considering iirst the three alcohols shown, water strongly raises the volatil ity of acetone with respect to methanol, raises; i it slightly relative to ethanol, and lowers it slightly relative to isopropanol. molecular weight would be less polar than isopropanol, and water would reducethe volatility of acetone with respect to such alcohols.

Alcohols of higher Propionaldehyde, n-butyraldehyde, isopropyl ether, and methyl ethyl ketone show substantial increases in volatility with respect to acetone upon the addition of water. This results apparently from the fact that these materials are less polar than acetone and thus are less compatible with water; this is also indicated by their incomplete miscibility with water. It is irnportant to note that for components which are normally lower boiling than acetone, such as` propionaldehyde, the addition of any quantity of Water improves the relative volatility for the separation. However, for materials higher boiling than acetone, such as n-butyraldehyde, the

addition of water in limited concentration nar- 'rows .the relative volatility and at a certain water concentration no separation whatever jcould be made. At higher water concentrations the relative volatility is reversed, and the separa- `tion again becomes possible.

desirable water concentration obviously depends on the exact nature of the impurities to be removed. In general it is desirable to feed to the Thus, the most water extractive distillation a crude acetone cut :from which higher boiling impurities have been removed by conventional distillation as complete. ly as possible. It is also desirable to operate the extractive distillation tower separating nonalcohols at a very high water concentration in the liquid phase, for example, to 99 mol per cent water, to insure adequate relative volatility reversal for higher-boiling non-alcohols. These high water concentrations have the further advantage that near the top of the rectication zone of the tower, where the impurities concentrate, the high water concentration helps to hold the. impurities in solution; if some oi the impurities form a separate liquid phase of low water concentration, the separation between impurities and acetone is more diicult than if a homogeneous water phase is maintained, since only in the water phase do the favorable relative volatility relationships exist.

It is also to be noted that at high water concentrations the three classes of compounds boiling close to acetone are ranked in the following relative volatility order: lowest, alcohols; intermediate, acetone; highest, other non-alcohols. The three types could be separated at the same water concentration, and in any desired order. That is, the non-alcohols and acetone could be separated from alcohols in the rst stage, and from each other in the second stage; or the acetone and alcohols could be separated from the non-alcohols in the iirst stage, and from each other in the second stage. Diierent water concentrations in each stage can also be employed.

TABLE I Relative volatilities of other organic compounds to acetone Approximate Water Concentration in Liquid Phase, Moi Compound Percent Methanol 1 l. 0 0.31 0. 28 0. 2 Ethanol 0. 45 0. 33 0. 33 0.33 Isopropauol 0. i3 0.46 0. 56 0. 6i Propionaldehyde.- l. 2 2 2. 3 2 2. 4 2. 5 N-Butyraldehyde. 0. 54 2 l. 4 2 l. 5 1 1.65 isopropyl Ether l l. 0 2 2 2 2. 5 2 3 Methyl Ethyl Ketone 0.48 1 l. 1 2 l. 2 2 1.4 Methyl Acetate l l. 0 i 3 2 4 1 6 N-Hexane 1 1.0 l l0 l35 1 50 1` Forms binary azeotrope with acetone. 2 Determined by comparison oi experimental relative volotlitics to ethanol.

Without attempting to explain the mechanism by which the desired separation occurs in the distillation column, it can be said that the process is one o f vapor-liquid extraction in which the liquid contains a greater concentration of water relative to the acetone being puriiied than under the normal Vfractional distillation conditions in the absence of the considerable amount of water internal reflux. It is evident from the results obtained that the water employed within the limits specified increases the effective vapor pressure of the impurities in comparison with the acetone being puried thus allowing the impurities to pass overhead from the distillation zone. The temperature of the crude acetone fed to the fractional distillation tower is preferably Aclose to the temperature of the liquid on the plate at the point of addition of the feed although it may be lower to partially condense vapors ously near the top of the column while the crude acetone being purified is continuously fed into the column at a lower point and while sufficient heat is provided to afford distillation throughout the column. The feed stream may be preheated to a temperature close to that of the internal liquid redux under equilibrium boiling conditions at the point of introduction. The preheated feed stream may be liquid, partially vaporized, or completely vaporized when introduced into the fractionation tower. Vapors of acetone and the impurities pass upwardly through the distillation zone in contact with descending internal liquid water reflux under equilibrium reboiling and reiluxing conditions. With adequate concentrations of water supplied as reflux, the impurities to be isolated distill overhead from the fractional distillation zone, while the acetone, in conjunction with any alcohols present, is removed from the bottom of the tower as an aqueous solution.

In using the high water concentration reuxes, some substances may be included in the water such as salts, e. g., phosphates, acetates, chlorides. Such substances frequently alter the relative volatilities between individual organic compounds in water solution. Even when no favorable effect is obtained on the relative volatility between individual organic compounds, the addition of highly polar compounds such as salts is advantageous because it generally increases the relative volatility between water and organic compounds. This effect reduces the heat consumption of the distillation by reducing the water content of the vapor stream in contact with a liquid reflux of given water concentration.

What is claimed is:

l. The method of separating acetone from a mixture thereof with n-butyraldehyde, at least one non-alcoholic impurity (A) selected from the group consisting of propionaldehyde, methyl acetate, dimethyl acetal, isopropyl ether and low 40 boiling saturated aliphatic hydrocarbons, and at least one alcoholic impurity (B) selected from the group consisting of methanol, ethanol and isopropanol which comprises continuously feeding the mixture to a. fractional distillation zone at an intermediate point thereof, continuously adding suicient water to the distillation zone at a point substantially above the mixture feed point to maintain an internal liquid water reflux having a water content in the range above mole percent below the point of addition of the water, distilling from the acetone feed mixture a vaporous mixture comprising n-butyraldehyde and the impurities wherein the distilled vaporous mixture ows countercurrent to the aqueous liquid reux, withdrawing n-butyraldehyde and impurity A as a distillate from the distillation zone and removing an aqueous solution of acetone and impurity B as bottoms from the fractional distillation zone. A

2. A process according to claim l in which the water content of the reiiux is maintained in the range above mole percent.

WILLIAM E. CATTERALL.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,929,901 Ricard et al Oct. 10, 1933 2,179,991 Bright et al. Nov. 14, 1939 2,198,651 BludWOrth Apr. 30, 1940 2,283,911 Brant et al May 26, 1942 2,290,442 Metzl July 21. 1942 2,290,636 Deanesly July 21, 1942 2,321,748 Hopkins June 15, 1943 2,379,110 Souders June 26, 1945 2,514,966 Pierotti et a1 July 1l, 1950 2,514,967 Pierotti et al July 11, 1.950 2,551,593 Gilliland et al May 8, 1951 2,551,625 Morrell et al May 8, 1951 OTHER REFERENCES 

1. THE METHOD OF SEPARATING ACETONE FROM A MIXTURE THEREOF WITH N-BUTYRALDEHYDE, AT LEAST ONE NON-ALCOHOLIC IMPURITY (A) SELECTED FROM THE GROUP CONSISTING OF PROPIONALDEHYDE, METHYL ACETATE, DIMETHYL ACETAL, ISOPROPYL ETHER AND LOW BOILING SATURATED ALIPHATIC HYDROCARBONS, AND AT LEAST ONE ALCOHOLIC IMPURITY (B) SELECTED FROM THE GROUP CONSISTING OF METHANOL, ETHANOL AND ISOPROPANOL WHICH COMPRISES CONTINUOUSLY FEEDING THE MIXTURE TO A FRACTIONAL DISTILLATION ZONE AT AN INTERMEDIATE POINT THEREOF, CONTINUOUSLY ADDING SUFFICIENT WATER TO THE DISTILLATION ZONE AT A POINT SUBSTANTIALLY ABOVE THE MIXTURE FEED POINT TO MAINTAIN AN INTERNAL LIQUID WATER REFLUX HAVING A WATER CONTENT IN THE RANGE ABOVE 80 MOLE PERCENT BELOW THE POINT OF ADDITION OF THE WATER, DISTILLING FROM THE ACETONE FEED MIXTURE A VAPOROUS MIXTURE COMPRISING N-BUTYRALDEHYDE AND THE IMPURITIES WHEREIN THE DISTILLED VAPOROUS MIXTURE FLOWS COUNTERCURRENT TO THE VAPOROUS LIQUID REFLUX, WITHDRAWING N-BUTYRALDEHYDE AND IMPURITIES A AS A DISTILLATE FROM THE DISTILLATION ZONE AND REMOVING AN AQUEOUS SOLUTION OF ACETONE AND IMPURITY B AS BOTTOMS FROM THE FRACTIONAL DISTILLATION ZONE. 